The present invention relates to an inspection method and apparatus using nuclear magnetic resonance for measuring a nuclear magnetic resonance signal from hydrogen, phosphorus or the like in a subject to be inspected. More particularly, the invention relates to a nuclear magnetic resonance imaging (MRI) apparatus for visualizing a nuclear density distribution, a relaxation time distribution, or the like and a data processing method in brain function measurement using the MRI apparatus (hereinafter, referred to as a FMRI: Functional Magnetic Resonance Imaging). Further in detail, the invention relates to a body motion correcting method of correcting positional deviation of the brain caused by physiological factors such as respiration and cardiac pulsation with high accuracy.
A target of an MRI which is presently used in clinics is proton, a main substance of the subject to be inspected. A spatial distribution of the proton density and a spatial distribution of a relaxation phenomenon of nuclear spins from an exciting state are visualized and two- or three-dimensional images of the shapes or functions of the human head, abdomen, legs and arms, and the like are acquired. An echo planar sequence (EPI: Echo Planar Imaging) of which an example is shown in FIG. 1, is a typical high-speed sequence of the MRI apparatus. According to the EPI, after the nuclear spins are excited by applying a radio frequency magnetic field, a first gradient magnetic field is switched at high speed and a second gradient magnetic field is repeatedly applied, thereby measuring all of echo signals necessary for image reconstruction.
Generally, a procedure for obtaining the MRI signal is shown by using trajectories in a frequency space (k-space) shown in FIG. 2 by persons skilled in the art. In the k-space, the axis of the abscissa denotes a readout gradient magnetic field application amount and the axis of the ordinate shows a phase encoding amount. The coordinate in the k-space is expressed by (kx, ky) and the center of the space is (0, 0). The trajectories in the k-space corresponding to a pulse sequence of FIG. 1 are as shown in FIG. 2. Data obtained by sampling one echo signal is arranged as complex data (a detection signal itself) on a trajectory along the axis of the abscissa in the k-space in accordance with data acquisition order. A data column of one echo signal exactly corresponds to one lateral row of the trajectory. The trajectory is shifted in the direction of the axis of the ordinate each time a phase encoding pulse is applied. In a single shot EPI, the k-space is scanned in a one stroke writing manner. Consequently, an EPI data set is formed. After performing a known inverting process to the EPI data set and a process for correcting phase distortion of the nuclear spin caused by eddy currents, a two-dimensional FFT is executed, thereby reconstructing an MR image (Japanese Patent Application Laid-Open No. 5-68674). Generally, in the process for correcting the phase distortion caused by the eddy currents in the EPI, a pre-scan is performed and eddy current reference data for removing the phase distortion caused by the eddy currents is acquired. A method of removing the phase distortion caused by the eddy currents from the EPI data set by using the eddy current reference data is employed (Japanese Patent Application Laid-Open No. 5-68674).
In recent years, an FMRI (functional MRI) for extracting local activation of the brain by a slight signal change in a local part of time-series MR images has been being put into practical use. The FMRI is a measuring method of extracting the activity of the brain by using images acquired in time series. That is, while applying stimuli such as light and sound to a subject for a predetermined period, tens or hundreds time-series images are acquired. After that, a region (hereinlater, called an activated region) in which the signal intensity is increased synchronously with the stimuli is extracted from the time-series images and the signal change before and after the application of the stimuli is observed. Since the signal change is microscopic and occurs locally, it is important to use an imaging sequence in which the signal change is detected very sensitively. It is also necessary to consider body motion of the subject during the measuring period. In the FMRI, MRI images before the application of the stimuli are used as reference images and the activated region is visualized by removing the reference images from time-series images after the application of the stimuli. The body motion therefore causes an erroneous extraction of the activated region. Conventional techniques of 1) image acquiring method and 2) body motion correcting method which are used in the FMRI will be described hereinbelow.
1) Image acquiring method used in the FMRI
One of the time-series image acquiring methods used in the FMRI is an EPI which can obtain one image within 100 ms. FIG. 3 shows another example of a sequence in the EPI. In the EPI, after the nuclear spins are excited by applying a gradient magnetic field 102 for selecting a slice to be visualized and a radio frequency (RF) magnetic field 101, a signal readout gradient magnetic field Gx 106 is switched at high speed and a phase encoding gradient magnetic field Gy 104 is repeatedly applied, and all of the echo signals necessary to reconstruct an image are measured. Reference numeral 103 denotes a pulse for giving an offset of a phase encoding and 105 indicates a pulse for giving an offset of the readout gradient magnetic field. The gradient magnetic field Gx is called a signal readout gradient magnetic field, the gradient magnetic field Gy is called a phase encoding gradient magnetic field, and a gradient magnetic field Gz is called a slice selecting gradient magnetic field. The echo signal has information of the magnitude, frequency, and phase. The nuclear spins are modulated by the application of the gradient magnetic fields Gx and Gy and position information is given as frequency and phase. However, the frequency is actually detected as a phase difference. As shown in FIG. 4, the measured echo signals are arranged in the frequency space (k-space) and the EPI data set is formed. Shown in FIG. 4 are a spatial frequency kx in the x direction; a spatial frequency ky in the y direction; and circles indicative of A/D sampling points. A distance between neighboring sampling points in the axis of abscissa kx corresponds to an application amount (time integral value of the magnitude of the magnetic field) of the signal readout gradient magnetic field Gx 106 which is applied during one sampling period. A distance between neighboring sampling points in the ky axis direction corresponds to an application amount (time integral value of the magnitude of the magnetic field) of the phase encoding gradient magnetic field Gy 104. The EPI data set is subjected to a 2-dimensional Fourier transformation, thereby reconstructing the image.
Although the EPI is an imaging method having an excellent time resolution, it also has a drawback such that the phase of the echo signal is disturbed by the eddy currents occurring by a high-speed switching of the strong signal readout gradient magnetic field Gx 106 and the quality of the reconstructed image is deteriorated. For solving this drawback, a method in which the phase of the EPI data set is corrected by performing a pre-scan in order to remove the phase disturbance of the echo signal caused by the eddy currents and measuring reference data for eddy current is disclosed in Japanese Patent Application Laid-Open No. 5-68674.
The eddy current reference data is measured without applying the phase encoding gradient magnetic fields Gy 103 and 104 in FIG. 3. An example of the correcting procedure is shown in FIG. 5. First, the reference data for the eddy current is gathered (step 21). A one-dimensional Fourier transformation in the kx direction is applied to the reference data for the eddy current (step 22). The phase variation in the kx direction after the Fourier transformation is calculated (step 23). The sign of the calculation result is inverted and the resultant data is memorized as a correction value (step 24). The processes from step 22 to step 24 are performed with respect to all of the columns in the k-space (step 25). Subsequently, the EPI data set is gathered (step 26). The 1-dimensional Fourier transformation in the kx direction is applied to the EPI data set (step 27). The phase variation in the kx direction after the Fourier transformation is calculated (step 28). The correction value obtained from the calculation in step 24 is added to the phase every corresponding column, thereby correcting the phase (step 29). The corrected data is 1-dimensional Fourier transformed in the ky direction (step 30), thereby reconstructing the image. With the above steps, the disturbance of the phase of the echo signal caused by the eddy currents is removed, so that the high quality of the EPI image can be achieved.
2) Body motion correcting method
There are, broadly speaking, two kinds of body motion which cause problems in the FMRI. The first body motion is motion of the whole head. The occurrence of the first body motion can be prevented by strongly fixing the head. Otherwise, a method of correcting the body motion by a signal process is disclosed in Japanese Patent Application Laid-Open No. 5-154130. According to the method, echo signals (hereinlater, called navigator echoes) for detecting the body motion are generated, and the body motion is detected and the correction value is derived. The second body motion relates to movement of the brain due to the change in the pressure in the cranium by respiration and cardiac pulsation. Since the second body motion is caused by physiological factors, the occurrence of it cannot be prevented. It is therefore necessary to reduce the influence by the body motion by a signal process. An example of correcting the second body motion by using the navigator echoes has been reported (Magn. Reson. Med. 31, 495-503 (1994)), in which a correction effect of reduction of signal fluctuations in the activated region is confirmed. In the FMRI, it is pointed out that slight motion of the target to be inspected becomes a problem when the signals are analyzed and a method of correcting the positional deviation between images due to the motion by a post process is disclosed (by Jun Omiya et al, "Correlation image process for MR function image", Medical Imaging Technology, Vol. 13, No. 4, pp. 583-584, July 1995). According to the method, the positional deviation between two images is detected and corrected by using information near the origin of the k-space.